The present invention generally relates to optical masks and methods of correcting defects in optical masks, and more particularly to a phase shift optical mask which is used in a projection exposure apparatus for producing semiconductor devices or the like, and to a method of correcting defects in such an optical mask.
In very large scale integrated circuits (VLSIs), it has become necessary to form patterns on the order of sub microns and half microns in order to realize a high operation speed and a high integration density.
For example, in the case of a dynamic random access memory having a memory capacity of 4 Mbits (hereinafter simply referred to as 4M-DRAM), the design rule is 0.8 .mu.m, and the design rule is 0.5 to 0.6 .mu.m in the case of a 16M-DRAM. The design rule in the case of a 64M-DRAM is about to become 0.3 .mu.m.
In order to realize such fine patterns, it is essential to realize a reduction projection exposure apparatus (so-called stepper) which is suited for mass production and provides a high resolution when using the lithography technique which forms the basis of the pattern forming process.
However, the reduction in the size of the designed fine patterns have progressed at a speed higher than the development of such exposure apparatuses. As a result, the size of the patterns which are now demanded is not obtainable by the nominal or rated resolution of the stepper. For this reason, attempts are being made to improve the resist material, the resist process or the optical mask in order to exceed the limit of the exposure apparatus.
As mentioned above, the improvement of the optical mask may be regarded as a means of improving the limit of the exposure apparatus. More particularly, the improved optical mask is the so-called phase shift optical mask which is locally provided with a phase shifter for inverting the phase of the exposure light. The lithography technique which uses such a phase shift optical mask is often referred to as a phase shift lithography technique.
As in the case of other lithography techniques, the phase shift lithography technique also requires a defect-free optical mask. However, since the conventional technique used to make the phase shift optical mask newly forms the phase shifter on the optical mask, the generation of defects is unavoidable.
Accordingly, in order to use the phase shift lithography technique which is effective for forming fine patterns when producing the VLSI, it is important to realize a phase shift optical mask having a construction which is less likely to generate defects and to realize a method of satisfactorily correcting the generated defects in a simple manner.
Generally, the defects generated in the phase shift optical mask which is employed by the phase shift lithography technique can be categorized into two kinds of defects. According to a first kind of detect, the phase shifter does not exist at a part of the optical mask where the phase shifter should exist or, even if the phase shifter does exist a portion of the phase shifter is missing. On the other hand, according to a second kind of defect, the phase shifter material is deposited at an unwanted part of the optical mask.
Next, a description will be given of an example of a conventional method of correcting a defect in a phase shift optical mask which is employed by the phase shift lithography technique, by referring to FIGS. 1 through 5. FIGS. 1 through 5 show cross sections of an essential part of the phase shift optical mask at essential stages of the correction. For the sake of convenience, it will be assumed that the phase shift optical mask has a light blocking pattern layer 2 made of Cr formed on a quartz substrate 1, and an etching stopper layer 3 which covers the light blocking pattern layer 2. In addition, a phase shifter 4 is formed on the etching stopper layer 3, but a defect is generated in this phase shifter 4. Accordingly, this defect is corrected by the following steps.
In FIG. 1, a defective portion 4A is generated in the phase shifter 4 when the phase shift optical mask is made by forming the phase shifter 4 which is made of SiO.sub.2 on the optical mask using the resist process of the lithography technique, evaporation and lift-off.
In FIG. 2, a resist layer 5 having an opening 5A which exposes the phase shifter 4 with the defective portion 4A using the resist process of the lithography technique.
In FIG. 3, the resist layer 5 is used as a mask to remove the defective portion 4A of the phase shifter 4 by etching.
In FIG. 4, evaporation is carried out with the resist layer 5 remaining, so as to form a SiO.sub.2 layer.
In FIG. 5, the structure is submerged in a resist removing solution so as to remove the resist layer 5 by lift-off, and the SiO.sub.2 layer formed in the previous step is patterned so as to form a new phase shifter 6.
Next, a description will be given of an example of a conventional method of correcting a defect different from the type described above in a phase shift optical mask which is employed by the phase shift lithography technique, by referring to FIGS. 6 through 9. FIGS. 6 through 9 show cross sections of an essential part of the phase shift optical mask at essential stages of the correction. For the sake of convenience, it will be assumed that the phase shift optical mask has a light blocking pattern layer 2 made of Cr formed on a quartz substrate 1, and an etching stopper layer 3 which covers the light blocking pattern layer 2. In addition, a phase shifter 4 is formed on the etching stopper layer 3, but the phase shifter material is deposited at a part of the optical mask where the phase shifter 4 should not exist. Hence, the phase shift material deposited at the unwanted part of the optical mask is removed by the following steps.
In FIG. 6, a residual portion 4B made of the phase shifter material is generated on the phase shift optical mask where no phase shifter should exist when the phase shift optical mask is made by forming the phase shifter 4 which is made of SiO.sub.2 on the optical mask using the resist process of the lithography technique, evaporation and lift-off.
In FIG. 7, a resist layer 5 having an opening 5A which exposes the residual portion 4B of the phase shifter 4 using the resist process of the lithography technique.
In FIG. 8, the resist layer 5 is used as a mask to remove the residual portion 4B of the phase shifter 4 by etching.
In FIG. 9, the structure is submerged in a resist removing solution so as to remove the resist layer 5, so that the phase shift optical mask only has the desired phase shifter 4 and not the residual portion 4B.
The defect correction described above in conjunction with FIGS. 1 through 5 can be made in one of two ways, as may be more easily understood by referring to the following the plan views.
FIG. 10 is a plan view showing an essential part of the phase shift optical mask for explaining the defect correction. In FIG. 10, those parts which are the same as those corresponding parts in FIGS. 1 through 5 are designated by the same reference numerals, and a description thereof will be omitted. In FIG. 10, the illustration of the etching stopper layer 3 is omitted so as to simplify the drawing.
In FIG. 10, a pattern 7 for providing a 0.degree. phase shift corresponds to a region where no Cr light blocking pattern layer 2 exists. On the other hand, a phase shifter 8 made of SiO.sub.2 provides a 180.degree. phase shift. This phase shifter 8 includes a defective portion 8A. In other words, a portion of the phase shifter 8 is missing at this defective portion 8A.
A first method of correcting this defective portion 8A shown in FIG. 10 is to remove the entire phase shifter 8 which has this defective portion 8A, and to form a new phase shifter which is required. But in this case, the entire phase shifter which is continuous for a relatively large area becomes the subject of the correction and it takes time and troublesome steps to extract from the original mask data the mask data which are required for the correction when making patterning process. Moreover, in order to guarantee the accurate positioning of the new phase shift layer, it requires an exposure apparatus having a high precision in order to carry out the necessary exposure for the correction.
In addition, if the pattern is long as in the case of a wiring pattern or a bit line or a word line of a memory, it not only takes a long time to make the necessary patterning for the correction, but there is a possibility that a new defect will be generated because the correction is carried out for a relatively large area or region.
On the other hand, the defective portion 8A shown in FIG. 10 may be removed by a second method which will be described hereunder with reference to FIGS.11 and 12. FIGS. 11 and 12 are plan views showing essential stages of the correction. In FIGS. 11 and 12, those parts which are the same as those corresponding parts in FIGS. 1 through 5 and 10 are designated by the same reference numerals, and a description thereof will be omitted. In FIGS. 11 and 12, the illustration of the etching stopper layer 3 is omitted so as to simplify the drawing.
In FIG. 11, a resist layer (not shown) having an opening 9 which exposes the defective portion 8A is formed using the resist process of the lithography technique. Then, this resist layer is used as a mask and a wet etching using buffered hydrofluoric acid (HF+NH.sub.4 F) as the etchant is carried out to remove a portion of the phase shifter 8 including the defective portion 8A.
Next, in FIG. 12, a SiO.sub.2 layer is formed on the entire surface by evaporation with the resist layer formed in the previous step remaining. Then, the entire structure is submerged in a resist removing solution so as to remove the resist layer by lift-off, and the SiO.sub.2 layer formed above is patterned to form a new regrown phase shifter portion 8B.
Accordingly, if the second method is used to correct the phase shifter 8 which partially includes the defective portion 8A, the phase shifter 8 and the new regrown phase shifter portion 8B will not be integrated. As a result, a portion which provides a 0.degree. phase shift, that is, a portion which provides a phase shift different from that provided by the phase shifter 8, is formed at a boundary portion between the phase shifter 8 and the new regrown phase shifter portion 8B. Therefore, a correct pattern exposure cannot be made on a wafer at a portion corresponding to the boundary portion.